The point at which Modified Configuration becomes viable on your average tank is when you can no longer have your ammorack damaged by any one shell. Your ammorack will get damaged as soon as its module HP is lower than the "repaired module hp" stat on that module.
Unfortunately this requires stat hunting. Some tanks have a really hard time achieving this. Others have no problems at all. For example, a Chrysler (280 ammorack hp, 190 repaired ammorack hp) with Modified Configuration basically wont get ammorack damaged ever again. A Liberte (250 ammorack hp, 200 repaired ammorack hp) needs Modified Config, Safe Stowage, and the Safe Stowage Directive to achieve the same thing.